The conventional techniques of the rewritable phase change type optical recording media and of the methods of recording on such recording media are described below.
These optical recording media have a recording layer mainly composed of an alloy of Te, Ge and Sb, etc. For recording, the recording layer is irradiated with focused laser beam pulses for a short time, to partially melt the recording layer. The molten portion is quickly cooled by thermal diffusion and solidified, to form recorded marks of the amorphous state. The light reflectance of the recorded marks is lower than that of the crystalline state and the recorded marks can be optically reproduced as recorded signals.
Furthermore, for erasure, the portion of the recorded marks is irradiated with a laser beam, to be heated at a temperature lower than the melting point of the recording layer and higher than the crystallization temperature, to crystallize the recorded marks of the amorphous state, for returning them into the original unrecorded state.
In the optical recording media with a Te alloy as the recording layer, the crystallization rate is high and high speed overwriting by a circular beam can be achieved by simply modulating the irradiation power into a recording power for writing the marks and an erasing power for erasing the recorded marks (T. Ohta et al, Proc. Int. Symp. on Optical Memory, 1989, p.49-50). These optical recording media with a recording layer usually have a heat-resistant and transparent dielectric layer each on both sides of the recording layer, to prevent the recording layer from being deformed or opened during recording. Furthermore, in another technique known, a reflection layer of a light-reflecting metal such as Al is provided on the side opposite to the beam incident side, to improve the signal contrast at the time of reproduction by optical interference effect and to allow easy formation of recorded marks of amorphous state by the effect of cooling the recording layer and also to improve erasabilities and overwrite cycles.
Especially the "rapid cooling structure", in which the recording layer and the dielectric layer between the recording layer and the reflection layer are kept as thin as about 20 nm, is considerably less lowered in recordability in spite of repeated overwriting and wider in the erasing power margin than the "slow cooling structure" in which the dielectric layer is as thick as about 200 nm. These rewritable phase change type optical recording media include optical discs. On the substrate of the optical disc, a groove is formed beforehand to form also land. At present, a general optical disc has a laser beam focused only on either the land or the groove, for recording and reproducing signals.
To increase the recording capacity of the optical disc, it is practiced to narrow the width of the land or groove, for shortening the track pitch. However, if the track pitch are shortened, the groove makes the angle of diffraction of reflected light large, to lower disadvantageously the tracking error signal for enabling the focused spot accurately to follow the track. Furthermore, if the width of the land or groove is narrowed, the width of the recorded pits is also narrowed, to lower the amplitude of reproduced signals as another problem. On the other hand, a technique of recording signals on both the tracks of the land and groove for increasing the recording capacity (JP-B 88-57859) is also known. However, if signals are recorded on both the tracks of the land and the groove, there are such problems (i) that the signal leak from the adjacent track (cross talk) increases, so that regenerated signals deteriorate, thus increasing the error, (ii) that since the difference in the amplitude of reproduced signals between the land and the groove becomes large, data detection is difficult and (iii) that at the time of recording, the marks of the adjacent track already recorded are erased (cross erase).
For such an optical disc, pit position recording has been used. However, in recent years, to meet the demand for higher density recording media, edge recording to allow higher density recording by recording information at edges of marks is going to be used instead of the pit position recording.
In edge recording, longer recorded marks must be formed than those of the pit position recording and at the rear portion of a long mark, the remaining heat effect of the recording layer widens the recorded mark, to deform disadvantageously the front-rear symmetry of the recorded mark like a tear drop. The deformation of the recorded mark deforms the reproduced signal, to increase jitter as a result.
As a means for solving this problem, it has been proposed to divide one recording pulse into a plurality of recording pulses (hereinafter called a pulse train) (JP-A 91-35425). Furthermore, a technique, in which a pulse corresponding to one half of the window margin and with a power smaller than the erasing power is applied for irradiation after the last pulse of each recording pulse train, is known (Proceedings of 5th Symposium on the Research of Phase Change Recording, p. 86, 1993).
However, in general, in a phase change type optical recording medium, the recorded marks of amorphous state are lower in reflectance and the difference in reflectance between the recorded amorphous state and the non-recorded crystalline state makes the difference in the photo-absorbed quantity of the recording layer large. So, depending on whether the state before overwriting is the crystalline state or the amorphous state with marks, the temperature rise during recording is different. Even in the edge recording using pulse trains, this phenomenon is liable to occur at the rear end of a long recorded mark since the temperature reached during recording is higher and it remarkably appears at a higher recording linear velosity. Thus, depending on the state before overwriting, the maximum temperature at the overwriting changes and the cooling rate of the recording layer also changes. So, a new recorded mark is modulated by the previous recorded mark, which is a factor to limit the jitter characteristic at the rear end of the mark and furthermore to limit erasability. That is, even if pulse trains are used, the deformation of recorded marks at the time of overwriting cannot be avoided.
Moreover, even if a pulse with a duration corresponding one half of the window margin and with a power smaller than the erasing power is added after the last pulse, the jitter characteristic at the rear end of the recorded mark at the time of overwriting cannot be sufficiently improved.